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Last Updated: , Created: Thursday, October 14th, 1999


In Canada, other than on the lower mainland of British Columbia and on Sable Island, it is impossible to maintain a comfortable level of humidity in the house and not have frost on single-pane windows most of the winter.

Using two panes of glass allows the inner pane to be warmer than the outer one. With control of the inside humidity, condensation or frost will not occur except when it becomes temporarily very cold.

Condensation on the inner surface of the inside pane of well-installed double-pane windows is generally considered to be the upper-limit indicator of how much humidity your house can tolerate on a given day. The colder it is outside, the colder is the inside pane of glass and the lower you will have to maintain your house's relative humidity. That's why household humidity levels are always based on outside temperatures. Triple-pane windows allow even higher humidity levels in the house without condensation, because the inner pane of glass is even warmer for the same temperature outdoors.

Frost on the inner pane also indicates that the outer pane has a serious leak, allowing cold outside air to get into the space between the windows and chill the inside pane. Frost on the frame usually indicates that the space between the finished window frame and the house rough framing may be insulated but is not sealed air tight, allowing cold wind to blow through and chill the frame itself.

Condensation or frost on the inner surface of the outside pane of glass is an indication that the inner pane leaks and warm, humid air from the room is leaking out and coming into contact with the cold outer pane. In this case you need to repair the weather-stripping on the inner pane of glass.

In short, when you see condensation, drop the humidity level. When you see frost on one pane of a double-pane window, reseal the pane that is clear. Click here to see the CMHC chart on proper indoor relative humidity as specific outddor temperatures.

Keywords: Windows, Environmental, Levels, Repair, Glass, Humidity, Condensation, Moisture, Frost, Frame, House, Relative Humidity, Temperature, Chart, Winter

Article 757